Whitman Richards

Stereopsis and Stereoblindness

SummaryPsychophysical tests reveal three classes of wide-field disparity detectors in man, responding respectively to crossed (near), uncrossed (far), and zero disparities. The probability of lacking one of these classes of detectors is about 30% which means that 2.7% of the population possess no wide-field stereopsis in one hemisphere. This small percentage corresponds to the probability of squint among adults, suggesting that fusional mechanisms might be disrupted when stereopsis is absent in one hemisphere.

Anomalous Stereoscopic Depth Perception

Normal or complete stereoscopic depth perception is based upon at least two and probably three mechanisms. These mechanisms may be isolated by studying depth judgments made by stereoanomalous individuals who are unable to discriminate disparities over wide ranges of disparity. The nature of the reductions observed among these stereoanomalous observers suggests that at least three ranges of the disparity are sampled in order to create three different pools of binocular activity. The pools correspond roughly to crossed, near-zero, and uncrossed disparities.

Spontaneous fixation tendencies for visual forms)

Haidinger’s brush was used as a method of locating fixation positions on a display. The various experimental patterns studied showed: (1) It is the already organized cortical representation of shape which governs fixation, rather than the peripheral input per se; (2) Acute angles near 20 deg are the most effective angular stimuli: (3) For figures subtending angles less than 5 deg, the eye is directed toward the center of the figure. and not toward its edge; and (4) Removing one segment from a completely enclosed figure may not alter the mean fixation position.

Structure from stereo and motion

Stereopsis and motion parallax are two methods for recovering three-dimensional (3D) shapes. Theoretical analyses of each method show that neither alone can recover rigid 3D shapes correctly unless other information, such as perspective and vertical disparity, is included. The solutions for recovering rigid structure from motion have a reflection ambiguity; the depth scale of the stereoscopic solution will not be known unless the fixation distance is specified in units of interpupil separation. (Hence the configuration will appear distorted.) However, the correct configuration and disposition of a rigid 3D shape can be recovered if stereopsis and motion are integrated, for then a unique solution follows from a set of linear equations. The correct interpretation requires only three points and two stereo views.

Two-dimensional curvature operators

Most computer vision systems use one-dimensional operators to calculate the curvature of boundaries or image contours. However, the more sophisticated biological systems use two-dimensional operators. Here we analyze how curvature can be computed by using two-dimensional operators and show the relation of this approach to the one-dimensional methods currently in use.

Color vision and image intensities: When are changes material?

The difficulty in understanding a biological system or its components without some idea of its goals has been emphasized by Marr. In this paper, a preliminary goal for color vision is proposed and analyzed. That goal is to determine where changes of material occur in a scene (using only spectral information). The goal is challenging because the effects of many processes (shadowing, shading from surface orientation changes, highlights, variations in pigment density) are confounded with the effects of material changes in the available image intensities. We show there is a minimal and unique condition, the spectral crosspoint, that rejects instances of these confounding processes. (If plots are made of image intensity versus wavelength from two image regions, and the plots intersect, we say that there is a spectral crosspoint.) An operator is designed to detect crosspoints; it turns out to resemble double-opponent cells described in primate visual cortex.

Encoding contour shape by curvature extrema.

Curvature extrema provide significant information about the shape of an image contour, such as a silhouette, and are the basis for the Hoffman-Richards codon representation for shape. This representation based on curvature easily translates into a binary string that will describe the abstract shape of any smooth image curve. The computation of the basic shape primitives requires dealing with two ever-pervasive problems: contour noise and scale. We show how contour noise can be estimated given knowledge of the shape of the filter used to compute curvature from the edge list of the contour. To handle the scale problem, we use an adaptation of Witkins scale space. Our algorithm differs from Witkins by using a notion of parts to set criteria for significant structures.

Disparity processing of spatial frequencies in man

1. Adaptation to a high‐contrast sine‐wave grating has been shown previously by Blakemore & Campbell (1969) to raise the modulation required to detect a low‐contrast grating that has the same or similar spatial frequency as the adapting grating.

Mechanisms of contour curvature discrimination.

Visual processing of contour curvature was investigated by measuring increment thresholds for curvatures from 0.31 to 25.4 deg-1. Curvature discrimination was assessed for three classes of stimuli: simple curved contours, high-frequency bandpass-filtered contours, and low-pass-filtered contours. High-frequency bandpass filtering had no effect on discrimination at low curvatures and only a modest effect at high curvatures. In contrast, low-pass filtering caused substantial threshold elevations at all curvatures. Thus the data lead to the surprising conclusion that high-spatial-frequency, orientation-selective mechanisms dominate curvature processing over the entire range of curvatures tested, a conclusion at odds with previous suggestions that large, low-spatial-frequency filters are involved in analyzing low curvatures. The data are explained accurately by a two-process model for curvature extraction: at high curvatures the local-processing model proposed by Wilson [J. Opt. Soc. Am. A. 2, 1191 (1985)] fits the data well, whereas at low curvatures orientations are compared at points displaced a fixed distance along the tangent to the curve.

Local versus global stereopsis: Two mechanisms?

Abstract Depth sensations are measured for a range of disparities and bar widths. The data show no evidence that stereopsis involves separate local and global modes of processing.